1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a DRAM made of a material of high dielectric constant and including a capacitor structure using the material.
2. Description of the Related Art
Development of DRAMs of higher density necessitates memory cells to be reduced in size and inevitably needs to keep a capacitor having capacitance at a specific degree. Materials of high dielectric constant for the capacitor and a capacitor structure using the materials as well as a stacked capacitor of a larger surface area and a thin film dielectric have been researched in this regard. A capacitor dielectric film of Ta.sub.2 O.sub.5 has been given attention now for the thin film dielectric as having high dielectric constant. But, the Ta.sub.2 O.sub.5 dielectric film when provided with a lower electrode using a polysilicon film have SiO.sub.2 naturally formed at an interface between Ta.sub.2 O.sub.5 and polysilicon, leading to a decrease of a practical dielectric constant of the Ta.sub.2 O.sub.5 dielectric film.
In case that the lower electrode uses W (tungsten), since W has a higher free energy in producing oxide than Ta (tantalum) to be low in oxidizing effect, a naturally oxidized WO.sub.x film is not likely to be produced at the interface of Ta.sub.2 O.sub.5 and W. Even when the WO.sub.x film is naturally produced at the interface, it does not behave as a dielectric. Hence, the decrease of dielectric constant of the Ta.sub.2 O.sub.5 film can be avoided. However, when using W for the lower electrode, a corresponding Ta.sub.2 O.sub.5 dielectric film is not provided with a uniform thickness in the course of processes for forming the same. It is difficult to avoid occurrence of leak current at the regions of locally thinner parts of the Ta.sub.2 O.sub.5 dielectric film. Hence, a capacitor structure built even with the material, the Ta.sub.2 O.sub.5 film, providing high dielectric constant does not achieve preferable high dielectric constant and resistance to leak current.